Generally speaking, a conventional magnetic motor includes two pieces that move relative to each other. Each of the two pieces includes some means of generating a magnetic field. The interaction between the magnetic fields generated by each of the pieces forces the pieces to move relative to each other. Usually, the magnetic field of at least one of the pieces will be selectively adjusted over time so that, as the relative spatial relationship of the pieces changes over time, the magnetic fields of the respective pieces will continue to interact to continue to activate relative motion in a desired direction.
Usually at least one of the pieces of the magnetic motor will employ one or more electromagnet(s), such as an electromagnetic coil, to generate its magnetic field(s). By using an electromagnetic piece, the timing of current supplied to the electromagnet(s) can be used to control the direction and strength of the magnetic fields with respect to time. By carefully controlling the electromagnetic piece's magnetic field as its counterpart piece moves, the magnetic field will pull and/or push the two counterpart pieces into relative motion. As the counterpart pieces continue in their relative motion, the direction and/or magnitude of the current in the electromagnet(s) can be changed so that the new magnetic field of the electromagnet(s) will continue to force the desired relative motion.
There are various geometries for magnetic motors. One popular geometry is the rotary motor. In the rotary motor, a rotor piece is driven to rotate relative to a stator piece. Although the scope of the present invention may include rotary motor embodiments, this document is primarily concerned with another popular conventional magnetic motor called the linear magnetic motor. Linear magnetic motors include a stator piece, and a shaft member that is driven to move linearly (that is, as a straight line translation) with respect to the stator piece.
More particularly, this document is primarily concerned with linear magnetic motors wherein an elongated shaft member: (1) is at least partially surrounded by the stator, and (2) is constrained to move linearly within the stator by a bearing. (Generally the bearing housing and stator are fixed relative to each other and can therefore be thought of as a subassembly.) As will be seen from the prior art embodiment described below, it is difficult to make a shaft that simultaneously: (1) performs well magnetically; and (2) performs well with respect to wear at the bearing.
FIGS. 1 and 2 show typical prior art linear magnetic motor 100, including shaft 102, stator 104 and bearings 106. Shaft 102 generates magnetic fields by virtue of having a series of built-in permanent magnets 110. Stator 104 generates magnetic fields through a series of annular magnetic coils 105. By timing the flow of current in the coils with respect to the position and/or momentum of shaft 102, the interaction of magnetic forces from the shaft and from the stator will actuate the shaft to move. More particularly, the shaft is constrained, by bearings 106, to move linearly in the direction of arrow D.
FIG. 2 shows a more detailed view of shaft 102 and one of the magnetic fields that it generates. Shaft 102 includes sleeve 109, annular, permanent magnet 110, pole pieces 112 and core 114. In this assembly, maximizing the magnetic force on the shaft will tend to advantageously maximize the thrust of the linear motor. In order to maximize the magnetic force on the shaft, the magnetic field of permanent magnet 110 should cause as much magnetic flux density as possible linking stator 104 and shaft pole pieces 112.
There are several variables that control the magnitude of the flux density in the vicinity of the stator. One variable is the strength of permanent magnet 110. For more thrust, the strength of magnet 110 should be increased as much as possible and/or as much as is cost effective. In practice, the magnets employed as annular, permanent magnets 110 tend to be extremely powerful permanent magnets. In fact, the permanent, annular magnets tend to be so powerful that the heavy shaft sub-assemblies often need to be handled with great caution. This is because of the tendency for the heavy shaft to be powerfully propelled through space due to the interaction between the powerful magnetic field of its own magnets 110 and any external magnetic field that may be present.
As shown in FIG. 2, another variable that has an influence on the flux density is the size of the effective air gap G. The effective air gap is the distance between pole piece 112 and stator 104. As shown in FIG. 2, the effective air gap G in this example is the sum of the actual air gap 108 and the thickness of sleeve 109. Some effective air gap is needed to prevent the shaft from rubbing against the non load-bearing surfaces of the stator poles. On the other hand, decreasing this air gap, without entirely eliminating it, will advantageously cause the field of magnet 110 to have greater flux density in the vicinity of the stator due to the increased proximity between magnet 110 and the stator. As flux density from magnet 110 in the vicinity of the stator increases, increased interaction of the magnetic fields results in increased force on the shaft, increased attendant actuation of the shaft and increased motor thrust.
Yet another variable affecting magnetic flux density in the vicinity of the stator is the flux density located along the effective air gap. As shown in FIG. 2, there are generally three paths A, B, C for the magnetic field of magnet 110. While magnet paths are generally circuits, it is noted that the magnetic “paths” referred to in this document refer to the portion of the magnetic circuit that lies outside of the magnet itself.
Path A passes through sleeve 109, which is part of the effective air gap. Path B passes through actual air gap 108, which is also part of the effective air gap. Path C passes through the stator. Permanent magnets are generally limited in the maximum amount of magnetic flux that they are capable of outputting. For an annular magnet of finite flux output capability, greater magnetic flux along paths A and B reduces the flux available for path C. As explained above, it is flux density of path C (that is, flux that reaches the vicinity of the stator) that contributes to motor thrust.
Shifting attention to the upper portion of FIG. 2, sleeve 109 is conventionally made from materials that: (1) have a low magnetic permeability; and (2) do not exhibit significant remanent magnetization. The non-magnetic nature of sleeve 109 works to minimize flux along sleeve 109 through path A. Nevertheless, some relatively small amount of magnetic flux is generally “lost” along path A. To represent this lost flux, a solitary dashed flux line is shown passing along and through the sleeve in FIG. 2. Because only a small fraction of the total flux is lost along path A, a higher portion of the total flux generated by magnet 110 will be directed through path C into the vicinity of the stator.
Not too much flux is “lost” at the actual air gap (that is, magnetic path B), either. Because actual air gap 108 is made of air, this potential flux leakage path B has extremely low permeability (the relative permeability of air equals 1.0) and no substantial remanent magnetization. Since the path B leakage flux is small and is primarily a result of sleeve 109, no dashed flux lines are shown along actual air gap 108 at the upper half of FIG. 2.
Because path A leakage flux is increased by the sleeve, one may question why sleeve 109 is present. One important reason for the sleeve is that the sleeve provides a bearing surface to slidably mate with bearing 106 as bearing 106 constrains the linear motion of shaft 102. If no sleeve were present, then the permanent magnets and the intermediate pole pieces of shaft 102 would contact the bearing. Because of the limited choice of materials that can be used to make the permanent magnets, and because of physical discontinuities between magnets and pole pieces, the exposed magnets would not generally provide an acceptable bearing surface. This is due to the friction and wear characteristics that a surface of exposed magnets and pole pieces would have. Therefore, a smooth, long-wearing sleeve is generally necessary at the outer major, bearing surface of a magnetic motor shaft.
Besides providing a relatively smooth and low-friction bearing surface, sleeve 109 also helps provide structural integrity for shaft 102. This can be especially important because the strong permanent magnets 110 can create magnetic attraction toward the stator wall sufficient to deform the entire shaft, absent proper structural support.
Therefore, sleeve 109 is a necessary evil of sorts. Preferably, under the conventional thinking, a material and thickness for the sleeve is selected to: (1) have a low magnetic permeability; (2) avoid magnetic saturation from the magnetic field of the shaft magnets; (3) have a low remanent magnetization value; (4) be easy to shape; (5) be relatively inexpensive; and/or (6) provide good bearing wear. In light of these various objectives, stainless steels are often used for shaft sleeves in magnetic motors. On the negative side, stainless steels are not the easiest materials to work with and do not necessarily provide the lowest rate of bearing wear. On the positive side, stainless steels do perform well relative to other materials that have low magnetic permeability and low residual magnetization. It is recognized that stainless steel is a metal with moderate wear characteristics, so sleeve 109 is constructed to be sufficiently thick to accommodate expected wear.